Spiky Bursts of Energetic Electron Beams Upstream of Nonstationary Quasi‐Perpendicular Shocks

Abstract Energetic electron beams play a crucial role in collisionless shocks by driving upstream plasma waves and providing seed populations for diffusive shock acceleration. However, the processes responsible for their generation at quasi‐perpendicular shocks remain poorly understood. Using two‐dimensional fully kinetic particle‐in‐cell simulations, we identify a new mechanism for generating electron beams at the leading edge of the shock foot. During shock reformation, reflected ions induce strong magnetic field curvature, causing ions and electrons to decouple dynamically. The resulting localized net charge separation generates large‐amplitude bipolar electric fields, which efficiently pick up and accelerate upstream electrons. We therefore refer to this novel mechanism as Reformation‐driven Electron Pickup Acceleration (REPA), which generates intense and spatially localized bursts of energetic electron beams. This process exhibits strong agreement with high‐resolution observations upstream of Earth's bow shock, and provides an unexpected acceleration pathway for thermal electrons.